These sensors are used for example in scanners. They comprise a bar of several parallel lines of photosensitive pixels; the sequencing of the control circuits of the various lines (the control of exposure time then of reading of the photogenerated charges) is synchronized relative to the relative travelling of the scene and of the sensor, so that all the lines of the sensor successively see one and the same line of the observed scene. The signals generated by each line are then added point by point for each point of the observed line.
At a constant exposure time, the sensitivity of the sensor is improved in the ratio of the number N of lines, or else, at constant sensitivity, the exposure time can be divided by N. This number N may be for example 16 or 32 for applications of industrial control or applications of earth observations from space, or even 60 to 100 lines for medical applications (dental, mammography, etc).
The signal-to-noise ratio is improved in the ratio of the square root of the number N of lines of the sensor.
Moreover, the non-uniformities of sensitivity of the pixels of one and the same bar, and the non-uniformities of dark current of the pixels, are diminished by the averaging that results from the addition of the signals of the various lines.
In charge-transfer image sensors (CCD sensors), the addition of the point-to-point signals is carried out simply by emptying from a line of pixels the charges generated and accumulated in the preceding line of pixels, in synchronism with the relative movement of the scene and of the sensor. The last line of pixels, having accumulated N times the charges generated by the observed image line, can be read.
The application of the CCD image sensors has the drawback of using high power supply voltages and consumes considerable power; this technology is based on the use of polycrystalline silicon gates that are adjacent and mutually overlapping; the density of integration is not very high.
The technology of the image sensors has subsequently evolved towards active-pixel sensors with transistors that are hereinafter called CMOS sensors for simplification because they are usually made using CMOS (complementary-metal-oxide-semiconductor) technology; in these CMOS sensors, there is no transfer of charges from line to line to a reading circuit or a register but there are active pixels with transistors that collect photogenerated electrical charges and convert them directly into a voltage or a current. The various lines of the sensor therefore successively supply voltages or currents representing the lighting received by the line. These structures do not make it possible to carry out noiseless accumulations of these currents or voltages; it is therefore difficult to produce a time delay integration sensor. The fabrication technology is nevertheless simple, it does not consume much power, and it operates at low voltage.
Attempts have however been made to produce CMOS time delay integration sensors.
Attempts have been made in particular to use switched capacitors in which successive received currents are integrated, thus accumulating on one and the same capacitor received charges of several pixels in a column (U.S. Pat. No. 6,906,749, WO0126382).
It has also been proposed to convert the signals originating from a line of pixels into digital values, to accumulate the digital value corresponding to the pixel of row j of the line in an accumulating register of row j which accumulates the digital values corresponding to the pixels of one and the same row j of N successive lines (patent FR2906080).
In patent FR2906081, it is proposed to apply to the photodiode of a pixel of a line the output voltage of a pixel of a preceding line in order to copy into it the charges of the preceding pixel before insulating the photodiode and integrating new charges due to the light, so that, at the end of an integration time, the photodiode comprises the sum of the charges corresponding to the preceding line and the new integrated charges. This operation however induces a transfer noise which deteriorates the signal-to-noise ratio.
Solutions using an accumulation of charges inside the pixel have been proposed, for example in patent publication US2008/0217661. They use a technology that is more complex than is strictly necessary to produce image sensors using CMOS technology, or else they have losses during the charge transfers.
Finally, in an earlier unpublished application by the applicant, it has been proposed to constitute pixels with an alternation of wide gates and narrow photodiodes separated from the gate by a region p++ at the potential of the substrate and forming a potential barrier preventing the charges from transferring except under very narrow gate fingers adjacent to the photodiode. The narrowness of the gate fingers creates, by the influence of the p++ regions that touch them, a potential barrier only when the gate is at low potential. When the gate is at high potential, this barrier lowers sufficiently. The directionality of the transfer is thus assured but at the price of a more complex structure and at the price of bottlenecks for the transfer of the charges.
The attempts to produce a time delay integration linear sensor using a technology that is simpler than the usual CCD technology have therefore not given complete satisfaction and one object of the invention is to propose another solution to these problems.